: Preterm lambs will be delivered and various ventilatory maneuvers performed (varying oxygen, tidal volume, and PEEP) to determine which particular strategy causes the least amount of lung injury. Next, preterm lambs will receive exogenous surfactant with different concentrations and combinations of apoproteins to determine if the injury process can be modified. Then, lambs will be exposed to endotoxin in utero to examine the impact of "inflammatory priming" on the development of lung injury. Finally, the ventilation and surfactant strategies that are found to cause the least injury will be tested to see if the arrest in lung development can be prevented.